It is known to utilize applanation tonometry to measure a patient's intraocular pressure. U.S. Pat. No. 3,585,849, issued on Jun. 22, 1971 to Bernard Grolman, discloses a non-contact tonometer which operates by discharging a fluid pulse of a known force-time relationship onto a cornea of a patient. The resulting deformation of the cornea from convexity through applanation to concavity, and return, is observed as a function of time and correlated to intraocular pressure.
Correlation of the observed deformation with intraocular pressure is carried out using Goldmann's calibration for applanation tonometry, which is based on a calibration mean corneal thickness of 0.52 mm, an approximation of the population mean corneal thickness of 0.522 mm. Since the population standard deviation from the mean population corneal thickness, 0.04 mm, is relatively small, clinical utility of applanation tonometry is preserved for the majority of patients. However, those patients having corneas lying beyond the first standard deviation of thickness are surely candidates for inaccurate intraocular pressure readings. For example, it has been reported in the American Journal of Ophthalmology, May 1993, Volume 115, pages 592-596, that for a true intraocular pressure of 20.0 mmHg measured by manometry, a corneal thickness of 0.45 mm produced an intraocular pressure underestimation of 4.7 mmHg by Goldmann applanation tonometry. Consequently, intraocular pressure measurements which do not account for corneal thickness are of compromised reliability as indicators of glaucoma.